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Weather routing is the discipline of selecting the optimum ocean-crossing route accounting for forecast wind, sea state, current, ice, and precipitation. The economic and safety differences between alternative routes on a Pacific or Atlantic crossing can be substantial — a well-routed voyage can save 12-36 hours + several tonnes of bunker versus a poorly-routed one, and can avoid the worst of a developing storm system that would otherwise punish the ship, crew, and cargo. Weather-routing decisions interact directly with charterparty performance clauses, hull & machinery insurance, and cargo-claim exposure.
A course of constant compass bearing between two points. Simple to navigate; appears as a straight line on a Mercator chart. NOT the shortest distance on a spherical earth — the rhumb-line is longer than the great-circle for any transocean route not on the equator or a meridian. Sometimes used for short passages or where great-circle navigation would incur difficulties (e.g. mid-ocean course changes).
The shortest surface path between two points on a sphere. Appears as a curve on a Mercator chart, bulging toward the pole. Requires periodic course changes (e.g. every 5° of longitude) to follow the great-circle track. Saves significant distance on transpacific + transatlantic voyages — Yokohama to Los Angeles is ~750 nm shorter by great-circle than rhumb-line. But: often passes through higher-latitude weather zones with more severe sea states.
The route computed to minimise total voyage cost/time given forecast weather + sea state + current. Typically deviates from great-circle by 100-500 nm to avoid a developing low-pressure system + take advantage of favourable currents. This is the market-standard approach for voyage-chartered vessels above ~10,000 DWT. Voyage cost is a weighted function of fuel + charter-hire + weather-damage risk + time-in-port bonus/penalty.
Provider services typically include: 3-5 day forecast + 10-day medium-range outlook; recommended route with waypoint list; 24/7 emergency advisory during storm development; post-voyage analysis + performance report. Cost: $500-$3,000 per voyage depending on ocean + service level.
The routing decision affects charterparty performance metrics differently depending on the charter type:
Tropical cyclones (hurricanes/typhoons/severe cyclonic storms) are the highest-severity weather hazard. TC seasonality by ocean basin:
Standard rule: maintain minimum 300 nm CPA (closest point of approach) from the centre of a Category 3+ TC. Weather routing providers issue daily advisories during TC development. National services (Japan Meteorological Agency, Joint Typhoon Warning Center, National Hurricane Center) provide free authoritative forecasts.
The Master has overriding authority for route selection under the master's absolute discretion clause of the applicable CP. The weather-router advises; the Master decides. In practice, most Masters follow router recommendations under normal conditions and reserve override authority for severe weather + parametric-roll + broaching risk. Documentation of Master's route-selection reasoning in the deck log is important for post-incident analysis + insurance defence.
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